Technical characteristics of the freezing refrigeration unit if 56. Determination of the characteristics of the refrigeration unit. Determination of the characteristics of the refrigeration system
Compressor type:
refrigeration piston, non-direct flow, single-stage, stuffing box, vertical.
Intended for work in stationary and transport refrigeration units.
Technical specifications , ,
| Parameter | Meaning |
| Cooling capacity, kW (kcal/h) | 12,5 (10750) |
| Freon | R12-22 |
| Piston stroke, mm | 50 |
| Cylinder diameter, mm | 67,5 |
| Number of cylinders, pcs | 2 |
| Crankshaft rotation speed, s -1 | 24 |
| Volume described by pistons, m 3 / h | 31 |
| Inner diameter of connected suction pipelines, not less than, mm | 25 |
| Inner diameter of connected discharge pipelines, not less than, mm | 25 |
| Overall dimensions, mm | 368*324*390 |
| Net weight, kg | 47 |
Characteristics and description of the compressor...
Cylinder diameter - 67.5 mm
Piston stroke - 50 mm.
Number of cylinders - 2.
The nominal shaft rotation speed is 24s-1 (1440 rpm).
The compressor is allowed to operate at a shaft rotation speed of s-1 (1650 rpm).
The described piston volume, m3/h - 32.8 (at n = 24 s-1). 37.5 (at n=27.5 s-1).
Type of drive - through a V-belt drive or clutch.
Refrigerants:
R12 – GOST 19212-87
R22- GOST 8502-88
R142-TU 6-02-588-80
Compressors are repairable products and require periodic maintenance:
Maintenance after 500 hours; 2000 h, including oil change and gas filter cleaning;
- Maintenance after 3750 hours:
- Maintenance after 7600 hours;
- average, repair after 22500 hours;
- major renovation after 45,000 hours
During the manufacturing process of compressors, the design of their components and parts is constantly being improved. Therefore, in the supplied compressor individual parts and the nodes may differ slightly from those described in the passport.
The operating principle of the compressor is as follows:
When the crankshaft rotates, the pistons return 
forward movement. When the piston moves downward in the space formed by the cylinder and the valve plate, a vacuum is created, the suction valve plates bend, opening holes in the valve plate through which refrigerant vapors pass into the cylinder. Filling with refrigerant vapor will occur until the piston reaches its lower position. As the piston moves upward, the suction valves close. The pressure in the cylinders will increase. As soon as the cylinder pressure becomes greater than the discharge line pressure, the discharge valves will open the holes in the ‘Valve Plate’ to allow refrigerant vapor to pass into the discharge chamber. Having reached the top position, the piston will begin to descend, the discharge valves will close and there will be a vacuum in the cylinder again. Then the cycle repeats. The compressor crankcase (Fig. 1) is a cast iron casting with supports at the ends for the crankshaft bearings. On one side of the crankcase cover there is a graphite oil seal, on the other side the crankcase is closed with a cover in which there is a block that serves as a stop for the crankshaft. The crankcase has two plugs, one of which serves to fill the compressor with oil, and the other to drain the oil. On the side wall of the crankcase there is a sight glass designed to monitor the oil level in the compressor. The flange in the upper part of the crankcase is intended for attaching the cylinder block to it. The cylinder block combines two cylinders into one iron casting that has two flanges: the upper one for connecting the valve plate to the block cover and the lower one for attaching to the crankcase. In order to protect the compressor and system from clogging, a filter is installed in the suction cavity of the unit. To ensure the return of oil accumulating in the suction cavity, a plug with a hole is provided connecting the suction cavity of the block to the crankcase. The connecting rod-piston group consists of a piston, connecting rod, 
finger sealing and oil scraper rings. The valve plate is installed in the upper part of the compressor between the cylinder blocks and the cylinder cover; it consists of a valve plate, suction and discharge valve plates, suction valve seats, springs, bushings, and discharge valve guides. The valve plate has removable suction valve seats in the form of hardened steel plates with two elongated slots in each. The slots are closed with steel spring plates, which are located in the grooves of the valve plate. The seats and plate are fixed with pins. The discharge valve plates are steel, round, located in the annular recesses of the plate, which are valve seats. To prevent lateral displacement, during operation the plates are centered by stamped guides, the legs of which rest against the bottom of the annular groove of the valve plate. From above, the plates are pressed to the valve plate by springs, using a common strip, which is attached to the plate with bolts on bushings. There are 4 pins fixed in the bar, on which bushings are placed that limit the rise of the discharge valves. The bushings are pressed against the valve guides by buffer springs. At normal conditions buffer springs do not work; They serve to protect the valves from damage due to hydraulic shocks in the event of liquid refrigerant or excess oil entering the cylinders. The valve plate is divided by the internal partition of the cylinder cover into suction and discharge cavities. In the upper, extreme position of the piston, there is a gap of 0.2...0.17 mm between the valve plate and the piston bottom, called linear dead space. The oil seal seals the outward drive end of the crankshaft. Oil seal type - graphite self-aligning. Shut-off valves - suction and discharge, are used to connect the compressor to the refrigerant system. An angled or straight fitting, as well as a fitting or tee for connecting devices, is attached to the shut-off valve body using a thread. When the spindle rotates clockwise, in its extreme position the spool closes the main passage through the valve into the system and opens the passage to the fitting. When the spindle rotates counterclockwise, in its extreme position it closes with a cone the passage to the fitting and completely opens the main passage through the valve into the system and blocks the passage to the tee. In intermediate positions, passage is open both to the system and to the tee. The moving parts of the compressor are lubricated by splashing. The crankpins of the crankshaft are lubricated through drilled inclined channels in the upper part of the lower connecting rod head. The upper head of the connecting rod is lubricated with oil that drains from inside bottom, piston and falling into the drilled hole in the upper head of the connecting rod. To reduce oil carryover from the crankcase, there is an oil removable ring on the piston, which dumps some of the oil from the cylinder walls back into the crankcase.
Amount of oil to be filled: 1.7 +- 0.1 kg.
See the table for cooling performance and effective power:
| Options | R12 | R22 | R142 | |
| n=24 s-¹ | n=24 s-¹ | n=27.5 s-¹ | n=24 s-¹ | |
| Cooling capacity, kW | 8,13 | 9,3 | 12,5 | 6,8 |
| Effective power, kW | 2,65 | 3,04 | 3,9 | 2,73 |
Notes: 1. Data are given in the following mode: boiling point - minus 15°C; condensation temperature - 30°C; suction temperature - 20°C; liquid temperature in front of the throttle device 30°C - for R12, R22 refrigerants; boiling point - 5°C; condensation temperature - 60 C; suction temperature - 20°C: liquid temperature in front of the throttle device - 60°C - for freon 142;
Deviation from the nominal values of cooling capacity and effective power is allowed within ±7%.
The difference between the discharge and suction pressures should not exceed 1.7 MPa (17 kgf/s*1), and the ratio of the discharge pressure to the suction pressure should not exceed 1.2.
The discharge temperature should not exceed 160°C for R22 and 140°C for R12 and R142.
Design pressure 1.80 mPa (1.8 kgf.cm2)
Compressors must maintain tightness when tested with an excess pressure of 1.80 mPa (1.8 kgf.cm2).
When operating on R22, R12 and R142, the suction temperature should be:
ts=t0+(15…20°С) at t0 ≥ 0°С;
tsun=20°С at -20°С< t0 < 0°С;
tsun= t0 + (35…40°С) at t0< -20°С;
Ministry of Education and Science of the Russian Federation
NOVOSIBIRSK STATE TECHNICAL UNIVERSITY
_____________________________________________________________
DEFINITION OF CHARACTERISTICS
REFRIGERATION UNIT
Guidelines
for FES students of all forms of study
Novosibirsk
2010
UDC 621.565(07)
Compiled by: Ph.D. tech. Sciences, Associate Professor ,
Reviewer: Dr. Tech. sciences, prof.
The work was prepared at the Department of Thermal Power Plants
© Novosibirsk State
Technical University, 2010
OBJECTIVE OF LABORATORY WORK
1. Practical consolidation of knowledge on the second law of thermodynamics, cycles, refrigeration units.
2. Familiarization with the IF-56 refrigeration unit and its technical characteristics.
3. Study and construction of refrigeration cycles.
4. Determination of the main characteristics, refrigeration unit.
1. THEORETICAL BASIS OF WORK
REFRIGERATION UNIT
1.1. Reverse Carnot cycle
A refrigeration unit is designed to transfer heat from a cold source to a hot one. According to Clausius's formulation of the second law of thermodynamics, heat cannot spontaneously transfer from a cold body to a hot one. In a refrigeration unit, such heat transfer does not occur by itself, but thanks to the mechanical energy of the compressor spent on compressing the refrigerant vapor.
The main characteristic of a refrigeration unit is the refrigeration coefficient, the expression of which is obtained from the equation of the first law of thermodynamics, written for the reverse cycle of the refrigeration unit, taking into account the fact that for any cycle the change in the internal energy of the working fluid D u= 0, namely:
q= q 1 – q 2 = l, (1.1)
Where q 1 – heat given to the hot spring; q 2 – heat removed from a cold source; l – mechanical work compressor.
From (1.1) it follows that heat is transferred to the hot source
q 1 = q 2 + l, (1.2)
a coefficient of performance is the fraction of heat q 2, transferred from a cold source to a hot one, per unit of compressor work expended
(1.3)
The maximum coefficient of performance value for a given temperature range between T mountains of hot and T cold heat sources have a reverse Carnot cycle (Fig. 1.1),
Rice. 1.1. Reverse Carnot cycle
for which the heat supplied at t 2 = const from the cold source to the working fluid:
q 2 = T 2 ( s 1 – s 4) = T 2 Ds (1.4)
and the heat given off at t 1 = const from the working fluid to the cold source:
q 1 = T 1 · ( s 2 – s 3) = T 1 Ds, (1.5)
In the reverse Carnot cycle: 1-2 – adiabatic compression of the working fluid, as a result of which the temperature of the working fluid T 2 gets higher temperature T hot spring mountains; 2-3 – isothermal heat removal q 1 from the working fluid to the hot spring; 3-4 – adiabatic expansion of the working fluid; 4-1 – isothermal heat supply q 2 from the cold source to the working fluid. Taking into account relations (1.4) and (1.5), equation (1.3) for the refrigeration coefficient of the reverse Carnot cycle can be presented as:
The higher the e value, the more efficient the refrigeration cycle and the less work l required for heat transfer q 2 from cold spring to hot.
1.2. Vapor compression refrigeration cycle
Isothermal heat supply and removal in a refrigeration unit can be achieved if the refrigerant is a low-boiling liquid whose boiling point at atmospheric pressure t 0 £ 0 oC, and at negative boiling temperatures the boiling pressure p 0 must be greater than atmospheric to prevent air leaks into the evaporator. low compression pressures make it possible to make a lightweight compressor and other elements of the refrigeration unit. With significant latent heat of vaporization r low specific volumes are desirable v, which allows you to reduce the size of the compressor.
A good refrigerant is ammonia NH3 (at boiling point t k = 20 °C, saturation pressure p k = 8.57 bar and at t 0 = -34 oC, p 0 = 0.98 bar). Its latent heat of vaporization is higher than that of other refrigerants, but its disadvantages are toxicity and corrosiveness towards non-ferrous metals, therefore ammonia is not used in household refrigeration units. Good refrigerants are methyl chloride (CH3CL) and ethane (C2H6); Sulfur dioxide (SO2) is not used due to its high toxicity.
Freons, fluorochlorinated derivatives of the simplest hydrocarbons (mainly methane), are widely used as refrigerants. The distinctive properties of freons are their chemical resistance, non-toxicity, lack of interaction with construction materials at t < 200 оС. В прошлом веке наиболее широкое распространение получил R12, или фреон – 12 (CF2CL2 – дифтордихлорметан), который имеет следующие теплофизические характеристики: молекулярная масса m = 120,92; температура кипения при атмосферном давлении p 0 = 1 bar; t 0 = -30.3 oC; critical parameters R12: p kr = 41.32 bar; t kr = 111.8 °C; v kr = 1.78×10-3 m3/kg; adiabatic exponent k = 1,14.
Freon production - 12, as destructive ozone layer substances that were banned in Russia in 2000; only the use of already produced R12 or extracted from equipment is permitted.
2. operation of the IF-56 refrigeration unit
2.1. refrigeration unit
The IF-56 unit is designed to cool the air in refrigeration chamber 9 (Fig. 2.1).
Fan" href="/text/category/ventilyator/" rel="bookmark">fan; 4 – receiver; 5 – condenser;
6 – filter drier; 7 – throttle; 8 – evaporator; 9 – refrigerator compartment
Rice. 2.2. Refrigeration cycle
In the process of throttling liquid freon in throttle 7 (process 4-5 V ph-diagram) it partially evaporates, but the main evaporation of freon occurs in the evaporator 8 due to the heat removed from the air in the refrigerating chamber (isobaric-isothermal process 5-6 at p 0 = const And t 0 = const). Superheated steam with a temperature enters compressor 1, where it is compressed by pressure p 0 to pressure p K (polytropic, actual compression 1-2d). In Fig. 2.2 also shows the theoretical, adiabatic compression of 1-2A at s 1 = const..gif" width="16" height="25"> (process 4*-4). Liquid freon flows into receiver 5, from where it flows through filter-drier 6 to throttle 7.
Technical data
Evaporator 8 consists of finned batteries - convectors. The batteries are equipped with a throttle 7 with a thermostatic valve. Capacitor 4 with forced air cooled, fan performance V B = 0.61 m3/s.
In Fig. 2.3 shows the actual cycle of a vapor compression refrigeration unit, built based on the results of its tests: 1-2a – adiabatic (theoretical) compression of refrigerant vapor; 1-2d – actual compression in the compressor; 2d-3 – isobaric cooling of vapors to
dew point t TO; 3-4* – isobaric-isothermal condensation of refrigerant vapor in the condenser; 4*-4 – condensate subcooling;
4-5 – throttling ( h 5 = h 4), as a result of which the liquid refrigerant partially evaporates; 5-6 – isobaric-isothermal evaporation in the evaporator refrigeration chamber; 6-1 – isobaric superheat of dry saturated steam (point 6, X= 1) up to temperature t 1.
Rice. 2.3. Refrigeration cycle ph-diagram
2.2. performance characteristics
The main operational characteristics of a refrigeration unit are cooling capacity Q, power consumption N, refrigerant consumption G and specific cooling capacity q. Cooling capacity is determined by the formula, kW:
Q = Gq = G(h 1 – h 4), (2.1)
Where G– refrigerant consumption, kg/s; h 1 – enthalpy of steam at the outlet of the evaporator, kJ/kg; h 4 – enthalpy of liquid refrigerant before the throttle, kJ/kg; q = h 1 – h 4 – specific cooling capacity, kJ/kg.
Specific is also used volumetric cooling capacity, kJ/m3:
q v = q/ v 1 = (h 1 – h 4)/v 1. (2.2)
Here v 1 – specific volume of steam at the outlet of the evaporator, m3/kg.
The refrigerant consumption is determined by the formula, kg/s:
G = Q TO/( h 2D – h 4), (2.3)
Q = c’pmV IN( t AT 2 - t IN 1). (2.4)
Here V B = 0.61 m3/s – performance of the fan cooling the condenser; t IN 1, t B2 – air temperature at the condenser inlet and outlet, ºС; c’pm– average volumetric isobaric heat capacity of air, kJ/(m3 K):
c’pm = (μ cpm)/(μ v 0), (2.5)
where (μ v 0) = 22.4 m3/kmol – volume of a kilomole of air under normal physical conditions; (μ cpm) – average isobaric molar heat capacity of air, which is determined by the empirical formula, kJ/(kmol K):
(μ cpm) = 29.1 + 5.6·10-4( t B1+ t AT 2). (2.6)
Theoretical power of adiabatic compression of refrigerant vapors in the process 1-2A, kW:
N A = G/(h 2A – h 1), (2.7)
Relative adiabatic and actual cooling capacities:
k A = Q/N A; (2.8)
k = Q/N, (2.9)
representing the heat transferred from a cold source to a hot one, per unit of theoretical power (adiabatic) and real ( electrical power compressor drive). The coefficient of performance has the same physical meaning and is determined by the formula:
ε = ( h 1 – h 4)/(h 2D – h 1). (2.10)
3. Refrigeration testing
After starting the refrigeration unit, you must wait until stationary mode is established ( t 1 = const, t 2D = const), then measure all instrument readings and enter them into measurement table 3.1, based on the results of which build a refrigeration unit cycle in ph- And ts-coordinates using the vapor diagram for freon-12 shown in Fig. 2.2. Calculation of the main characteristics of the refrigeration unit is performed in table. 3.2. Evaporation temperatures t 0 and condensation t K is found depending on pressure p 0 and p K according to table 3.3. Absolute pressures p 0 and p K is determined by the formulas, bar:
p 0 = B/750 + 0,981p 0M, (3.1)
p K = B/750 + 0,981p KM, (3.2)
Where IN – Atmosphere pressure according to barometer, mm. Hg Art.; p 0M – excess evaporation pressure according to the pressure gauge, atm; p KM – excess condensation pressure according to the pressure gauge, atm.
Table 3.1
Measurement results
Magnitude | Dimension | Meaning | Note |
|
Evaporation pressure p 0M | by pressure gauge |
|||
Condensation pressure p KM | by pressure gauge |
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Temperature in the refrigerator compartment, t HC | by thermocouple 1 |
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Refrigerant vapor temperature in front of the compressor, t 1 | by thermocouple 3 |
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Refrigerant vapor temperature after the compressor, t 2D | by thermocouple 4 |
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Condensate temperature after the condenser, t 4 | by thermocouple 5 |
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Air temperature after the condenser, t AT 2 | by thermocouple 6 |
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Air temperature in front of the condenser, t IN 1 | by thermocouple 7 |
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Compressor drive power, N | by wattmeter |
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Evaporation pressure p 0 | according to formula (3.1) |
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Evaporation temperature t 0 | according to table (3.3) |
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Condensation pressure p TO | according to formula (3.2) |
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Condensation temperature t TO | according to table 3.3 |
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Enthalpy of refrigerant vapor before the compressor, h 1 = f(p 0, t 1) | By ph-diagram |
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Enthalpy of refrigerant vapor after the compressor, h 2D = f(p TO, t 2D) | By ph-diagram |
|||
Enthalpy of refrigerant vapor after adiabatic compression, h 2A | By ph- diagram |
|||
Enthalpy of condensate after the condenser, h 4 = f(t 4) | By ph- diagram |
|||
Specific volume of steam in front of the compressor, v 1=f(p 0, t 1) | By ph-diagram |
|||
Air flow through the condenser V IN | By passport fan |
Table 3.2
Calculation of the main characteristics of the refrigeration unit
Magnitude | Dimension | Meaning |
||
Average molar heat capacity of air, (m Withpm) | kJ/(kmol×K) | 29.1 + 5.6×10-4( t B1+ t AT 2) | ||
Volumetric heat capacity of air, With¢ pm | kJ/(m3×K) | (m cp m) / 22.4 | c¢ p m V IN( t AT 2 - t IN 1) | |
Refrigerant consumption, G | Q TO / ( h 2D – h 4) | |||
Specific cooling capacity, q | h 1 – h 4 | |||
Cooling capacity Q | Gq | |||
Specific volumetric refrigeration capacity, qV | Q / v 1 | |||
Adiabatic power, N a | G(h 2A – h 1) | |||
Relative adiabatic cooling capacity, TO A | Q / N A | |||
Relative real cooling capacity, TO | Q / N | |||
Refrigeration coefficient, e | q / (h 2D – h 1) |
Table 3.3
Freon-12 saturation pressure (CF2 Cl2 – difluorodichloromethane)
1. Diagram and description of the refrigeration unit.
2. Tables of measurements and calculations.
3. Completed task.
Exercise
1. Construct a refrigeration cycle in ph-diagram (Fig. A.1).
2. Make a table. 3.4, using ph-diagram.
Table 3.4
Initial data for constructing a refrigeration unit cycle ints -coordinates
2. Construct a refrigeration cycle in ts-diagram (Fig. A.2).
3. Determine the value of the refrigeration coefficient of the reverse Carnot cycle using formula (1.6) for T 1 = T K and T 2 = T 0 and compare it with the coefficient of performance of a real installation.
LITERATURE
1. Sharov, Yu. I. Comparison of cycles of refrigeration units using alternative refrigerants // Energy and heat power engineering. – Novosibirsk: NSTU. – 2003. – Issue. 7, – pp. 194-198.
2. Kirillin, V. A. Technical thermodynamics / , . – M.: Energy, 1974. – 447 p.
3. Vargaftik, N. B. Handbook of thermophysical properties of gases and liquids / . – M.: science, 1972. – 720 p.
4. Andryushchenko, A. I. Fundamentals of technical thermodynamics of real processes / . – M.: graduate School, 1975.
The IF-56 unit is designed to cool the air in refrigeration chamber 9 (Fig. 2.1). The main elements are: freon piston compressor 1, air-cooled condenser 4, throttle 7, evaporative batteries 8, filter-drier 6 filled with a desiccant - silica gel, receiver 5 for collecting condensate, fan 3 and electric motor 2.
Rice. 2.1. Diagram of the IF-56 refrigeration unit:
Technical data
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Compressor brand |
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Number of cylinders |
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Volume described by pistons, m3/h |
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Refrigerant |
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Cooling capacity, kW |
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at t0 = -15 °С: tк = 30 °С |
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at t0 = +5 °С tк = 35 °С |
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Electric motor power, kW |
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Outside surface capacitor, m2 |
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External surface of the evaporator, m2 |
Evaporator 8 consists of two finned batteries - convectors. The batteries are equipped with a throttle 7 with a thermostatic valve. 4 forced air cooled condenser, fan performance
VB = 0.61 m3/s.
In Fig. 2.2 and 2.3 show the actual cycle of a vapor compression refrigeration unit, built based on the results of its tests: 1 – 2a – adiabatic (theoretical) compression of refrigerant vapor; 1 – 2d – actual compression in the compressor; 2d – 3 – isobaric cooling of vapors to
condensation temperature tk; 3 – 4* – isobaric-isothermal condensation of refrigerant vapor in the condenser; 4* – 4 – condensate subcooling;
4 – 5 – throttling (h5 = h4), as a result of which the liquid refrigerant partially evaporates; 5 – 6 – isobaric-isothermal evaporation in the evaporator of the refrigeration chamber; 6 – 1 – isobaric superheating of dry saturated steam (point 6, x = 1) to temperature t1.
